This invention relates to a power delivery system having a continuously variable ratio transmission such as might be used in an automotive vehicle and, more particularly, to a microprocessor controlled system and control method for reducing the fuel flow to the system prime mover upon a commanded decrease in power delivery. The present invention represents an improvement in the control system disclosed in pending and commonly assigned application Ser. Nos. 380,922 and 380,923 filed May 21, 1982 and which are incorporated herein by reference.
The quest for greater fuel economy of automotive vehicles has led to significant improvements in engine and transmission design and control. Continuously variable ratio transmissions (CVT) have shown particular promise in this regard. It will be appreciated that at any given vehicle speed, and for any needed propulsive force, a certain transmission ratio will provide maximum fuel economy for a given engine. In addition, for any given vehicle speed, one transmission ratio will permit maximum acceleration with that engine. Since a CVT with the proper ratio range can provide any desired transmission ratio, it is obviously attractive for automobiles from the standpoint of economy, low emissions and performance. If the mechanical efficiency of the CVT is high and its ratio range is wide enough, it can even be possible to have both maximum economy and maximum performance in the same vehicle. Among the obvious benefits are fully automatic operation, smooth, stepless and rapid response to driver demand and quieter cruising.
Many different CVT configurations have been developed in the prior art. These include, for example, hydrostatic transmissions; rolling contact traction drives; overrunning clutch designs; electrics; multi-speed gear boxes with slipping clutch; and V-belt traction drives. Of these the V-belt traction drives appear attractive for small to medium size passenger car applications because of their compactness, lightness and simplicity of design. Basically, this type of CVT comprises a V-belt which interconnects a driver sheave and a driven sheave, the diameters of the sheaves being variable to change the ratio of the CVT. Recent advances in belt design have resulted in improved belt durability and longevity. If sheave movement can be properly controlled so as to avoid undue stresses on the belt, it is expected that a very long belt life can be achieved.
Many control schemes have been devised for engine-CVT systems in attempts to maximize fuel economy. These have been based on empirical analyses of individual engine performance, and the realization that, for any desired power output, there is an optimum combination of engine speed and torque which will result in minimum fuel consumption. This is illustrated in FIG. 1.
FIG. 1 is a typical performance map of a four cylinder spark ignition passenger car engine having a displacement of approximately 2.5 liters. The map is a plot of engine torque T.sub.E and brake horsepower B.sub.HP as a function of engine speed N.sub.E. The dot-dash line near the top of the map is a plot of engine torque at full throttle. The series of curves in solid black lines are fuel consumption contours, indicating constant brake specific fuel consumption (BSFC) IN 1 b.M/BHP-hr. Minimum fuel consumption occurs at a point designed by 0.4 pounds per horsepower-hour. The series of dashed lines indicates power output of the engine. The ideal operating line for low fuel consumption is indicated by the heavy solid line f(N.sub.E), this curve being a function of engine speed. The ideal operating line for low fuel consumption is purely a function of engine characteristics and is optimal regardless of vehicle road speed. Other ideal operating lines may appear on the performance map, for example, the ideal operating line for low emissions.
In a vehicle with a conventional, manually shifted gearbox, forward speed ratios usually are available in only four or five steps. The operating point of the engine on the performance map is determined by drive shaft speed, power or torque commanded, and transmission gear ratio. Since there are only a few gear ratios available in a typical transmission, the engine must be throttled much of the time. The engine must therefore operate most of the time at high BSFC values. In contrast, a CVT is able to vary its speed ratio continuously to allow the engine to run at wider throttle and lower BSFC values.
Perhaps the most difficult task demanded of a control system for an engine-CVT system is to maintain engine operation along the ideal operating line. This is due to the almost continuous transient nature of operation of an automotive vehicle, there being hardly ever a time when road load and commanded torque or power remain constant. Transient conditions usually are dealt with by a change in CVT ratio, engine speed and throttle. Prior art control systems, by their very nature, permit an excursion of engine operation away from the ideal operating line before returning back to it at steady state. An example of such an excursion is shown in FIG. 1 by dashed line X-Y-Z. The result is that engine operation approaches, but hardly ever is maintained on the ideal operating line. Two such prior art systems are illustrated in FIGS. 2 and 3.
FIG. 2 schematically illustrates a system devised by Peter Stubbs for British Leyland. This system is described in greater detail in Stubbs, The Development of a Perbury Traction Transmission for Motor Car Applications, ASME Publication No. 80-C2/DET-59 (August, 1980). In this system, engine speed, throttle position and CVT ratio signals are all fed to a computer controller which has, in its memory, the engine operating characteristics for minimum fuel consumption. The computer controller generates, as a function of these variables, an engine control signal for adjusting the position of the throttle, and a ratio rate signal which changes the ratio of the CVT. The throttle is under the direct control of the vehicle accelerator pedal so that, while the engine control signal may vary the throttle position somewhat from that commanded by the driver, the throttle position still is primarily a function of commanded power or torque.
FIG. 3 is a schematic representation of the system devised by Miyao for Aisin Seiki. This system is described in greater detail in U.S. Pat. No. 4,091,690. Here, as in the Stubbs system, engine throttle is primarily a function of commanded power or torque by direct connection with the accelerator pedal. The computer generates a ratio rate signal to change the CVT ratio as a function of measured throttle position and engine torque and speed. Inherently sensed output torque also affects the CVT ratio.
In these, as well as in virtually all other engine-CVT control systems, throttle position is controlled directly by the vehicle accelerator pedal, or is a direct function of pedal position, as well as other parameters. Engine and transmission control usually are directly related to one another. Such control schemes permit engine operation during transients to vary from the ideal operating line. Excursions away from the ideal operating line result in less than optimum engine operation (e.g., excessive fuel consumption, or excessive emissions), until effective control is resumed by the system during steady state operation. As pointed out earlier, however, most vehicular operation is transient in nature, rather than steady state, so that substantially all engine operation occurs off the ideal operating line. Emissions calibrations must therefore be made in a substantial portion of the engine performance map. Most prior art control systems also must be specifically tailored to particular engines. This requires numerous specially designed control systems for a fleet of differently powered vehicles. In addition, most prior art control systems cannot compensate for varying engine conditions, the result being vehicle drivability which varies with engine temperature, state of tune, age and alititude. Close duplication of conventional vehicle characteristics also is a problem with prior art CVT control schemes.
The above noted disadvantages and deficiencies of prior art CVT control schemes were overcome by the control system and method disclosed in the pending prior applications mentioned above. These application disclose that engine operation can readily be maintained along the ideal operating line by providing for totally independent engine and transmission control. That is, the position of the engine throttle is totally independently of accelerator pedal position. Throttle position and, hence, engine output torque simply is a function of engine speed only, and that function may be any desired relationship, for example, the ideal operating line for low fuel consumption, the ideal operating line for low emissions, or a compromise ideal operating line for low fuel consumption and low emissions. Torque, power or other desired performance parameters commanded by the accelerator pedal controls CVT ratio, and engine speed is determined by the load placed thereon, which is a function of road load and CVT ratio. Hence, throttle position is precisely adjusted in accordance with the ideal function for any load placed on the engine. With the control system and method disclosed in the prior applications, anomalous engine and vehicle behavior, such as engine overspeed and underspeed conditions, can be prevented, transient start-up from rest can be accommodated, and the vehicle can be made to perform almost in all respects just as a vehicle with a conventional automatic transmission.
For convenience, the control system and method disclosed in the prior applications and the improvement therein provided by the present invention, are described throughout this specification in the context of an engine-CVT propulsion system for an automotive vehicle. It should be understood, however, that the principles disclosed are equally applicable to any type of power delivery system, including but not limited to other vehicular systems using internal or external combustion engines of any design, or to stationary power plants for driving compressors, generators or any other type of machinery. Where the term "throttle" is used, the term is understood to encompass any mechanism for controlling the delivery of fuel to the engine or other prime mover, be it a conventional carbureted spark-ignition engine wherein fuel flow varies with throttle butterfly position, a fuel injected spark-ignition or diesel engine, a gas turbine, and so on.
Referring to FIGS. 4-11, the control system disclosed in the pending prior applications will now be discussed. FIG. 4 illustrates the functional relationship of the components used to implement the control system. An engine 10 is drivingly coupled to a continuously variable ratio transmission (CVT) 14 through a clutch or fluid coupling (not shown). Fuel is fed to engine 10 by a fuel delivery means 12, which may be the throttle and fuel jets of a conventional carburetor, a fuel injection system or the like. CVT 14 may be one of the many types of continuously variable ratio transmissions discussed above. Output shaft 16 delivers power and torque from the engine and CVT. The ratio of the CVT is set by a CVT ratio controller 17, which generates a rate of change of ratio signal kR as a function of output torque T.sub.O measured by torque sensor 19 and commanded power or torque commanded by accelerator pedal 18. Other parameters indicative of engine-CVT system performance may be used by ratio controller 17 to effect a change of CVT ratio in a similar manner. For example, rather than using desired output power or torque and measured actual output torque, commanded and measured vehicle acceleration, output shaft accelerator, or other parameters could be used. In the control system shown in FIG. 4, however, CVT ratio is strictly a function of commanded power or torque and measured output torque, and is completely independent of engine operation. Engine control, on the other hand, is provided by an engine controller 100 which adjusts fuel delivery means 12 in accordance with measured engine speed N.sub.E. This relationship may desirably be the ideal engine operating line for low fuel consumption, the ideal operating line for low emissions, a compromise of the two, or any other desired engine operating characteristics.
FIG. 5 schematically illustrates the entire control system in greater detail. The particular type of CVT illustrated in FIG. 5 is the variable diameter pulley, V-belt traction drive type having a driven sheave 20 connected to output shaft 16 and a driver sheave 30 which is coupled to engine 10. Belt 15 interconnects sheaves 20 and 30 to transmit motive power therebetween. Sheaves 20 and 30 are hydraulically actuated by pressurized fluid to vary the driving diameters. Sheave 20 has an axially fixed portion 22 and an axially movable portion 24. Pressurized fluid in a fluid chamber 26 behind movable portion 24 provides the axial force required to maintain portions 22 and 24 at a fixed distance from one another (i.e., to hold the driving diameter of sheave 20 constant), and to move portion 24 toward or away from portion 22 to vary the driving diameter. Simlarly, sheave 30 has an axially fixed portion 32 and a movable portion 34 which is under the influence of fluid pressure in chamber 36. Proper pressures in chambers 26 and 36 to keep belt 15 under proper tension are maintained by the control system, as described below.
The position of throttle (fuel delivery means) 12 is controlled by a throttle servo 13 which receives signals from engine control circuit 100. During certain transient operations (described below) fuel delivery may be diminished by a fuel diminishing valve 11, or fuel delivery may be suspended completely by a fuel suspension mechanism 9. The fuel diminishing and suspension functions may be performed, for example, by a single solenoid valve operable in variable modes. Engine control circuit 100 is responsive to inputs from the accelerator pedal (.alpha.), engine speed (N.sub.E), a manual override switch which permits operation in the automatic or manual mode, and a start/neutral switch (S/N) which insures that the vehicle will remain stationary when the engine is started.
Fluid pressure for activating the driven sheave is provided by a sheave pressure generator 200 which acts through a pressure servo controller 250 and a fluid distribution circuit 500. Similarly, fluid pressure for activating the driver sheave 30 is provided by sheave pressure generator 300 acting through a servo controller 350 and fluid distribution circuit 500. Pressure generator 200 is responsive to inputs of engine speed N.sub.E, accelerator position .alpha., drive shaft speed N.sub.DS measured by a sensor associated with drive shaft 16, and CVT ratio R. Ratio R is generated by CVT ratio circuit 600 and is the quotient of engine speed NE divided by drive shaft speed NDS.
A starting clutch 40 is provided which couples engine 10 and CVT 14. Clutch 40 is disengaged when the vehicle is stationary, and is partially engaged during slow vehicle operation, gradually approaching full engagement, which occurs as described below at a predetermined point of operation. Starting clutch 40 is controlled by a control circuit 400 which is responsive to accelerator pedal position .alpha., engine speed N.sub.E and the auto/manual switch, through servo controller 450 and fluid distribution circuit 500.
FIGS. 7, 8 and 9 schematically illustrate in greater detail the functional relationships of the components illustrated in FIG. 5. FIG. 7 is primarily directed to the engine control circuit 100. A key element of control circuit 100 is function generator 102, which may generate a function representative of any desired engine operating characteristic. For this embodiment the function f(NE) is chosen as the ideal engine operating line for low fuel consumption. .theta. represents throttle angle, which is proportional to desired engine output torque. FIG. 1 graphically illustrates this function as f(N.sub.E). The value of the function produced by generator 102 is fed directly to throttle servo 13 via amplifier 104. In the event the automatic control system is disabled, it is possible to switch to a manual mode through mode switch 106. In the manual mode, accelerator position .alpha. is directly communicated to throttle servo 13 via amplifier 104. The start/neutral switch S/N also operates through mode switch 106.
A fuel suspension comparator 108 provides for engine overspeed control, which may tend to occur upon vigorous acceleration or if there is a malfunction in the control system. Comparator 108 compares engine speed N.sub.E to the maximum permissible engine speed, for example, 6000 rpm. If N.sub.E is greater than 6000 rpm, fuel suspension mechanism 9 is activated to suspend delivery of fuel to engine 10. Fuel suspension mechanism 9 may be, for example, a solenoid cutoff valve.
Another engine speed control is provided to counteract the inherent tendency of the vehicle to speed up when the accelerator pedal is released. This phenomenon occurs upon deceleration because the vehicle inertia becomes coupled to the inertia of a relatively unthrottled engine through a transmission whose ratio is changing towards overdrive. This undesirable tendency is even more pronounced when the accelerator pedal is released suddenly and completely. This anomalous behavior is prevented by reducing fuel flow to the engine when pressure on the accelerator pedal is relieved, the reduction of fuel flow being proportional to the rate at which pedal position decreases (-.alpha.), and by reducing fuel flow even further when the accelerator pedal position drops to below 3.6% of full excursion. To accomplish this control, a pulse width modulator 110 controls fuel diminishing valve 11, the duty cycle (i.e., the percentage of the pulse cycle during which fuel diminishing valve is held open) of modulator 110 being inversely proportional to the rate at which pedal position .alpha. decreases (-.alpha.). -.alpha. is derived from a differentiator 112 only if .alpha. is less than zero. In addition, a fuel diminishing comparator 114 reduces the duty cycle of modulator 110 to or near zero when pedal position .alpha. drops to below 3.6%.
FIG. 8 relates primarily to the starting clutch control circuit 400. It will be appreciated that some type of coupling must be provided between the engine and the CVT in order to permit the engine to idle while the vehicle is stationary. A fluid coupling could be used, but the mechanical losses inherent in such a device are antithetical to the desired objective of maximizing fuel economy. A torque converter with a lock-up clutch would be an improvement, but a mechanical clutch is preferred, and one which is hydraulically actuated would be well suited for this purpose. The object here, as in the conventional automobile, is to totally disengage the clutch when the vehicle is stationary, and to gradually engage it to begin vehicle movement and progressively engage the clutch further as the vehicle speed increases. To this end the measured transmission ratio R (which is computed as the quotient of engine speed N.sub.E and drive shaft speed N.sub.DS by ratio circuit 600) is fed to a comparator 402. Comparator 402 closes switch 404 when R exceeds 4.7 to deliver the signal from amplifier 406 to throttle servo 13 via amplifier 104. This signal is equal to .alpha.-N.sub.E', where N.sub.E ' is a function produced by generator 408 equal to K(N.sub.E -1000 rpm). Thus, the accelerator pedal 18 is coupled directly to throttle 12 in a variable way defined by .alpha.-N.sub.E '. The constant K is selected such that engine speed cannot exceed 2500 rpm if the clutch is not fully engaged. This direct coupling of accelerator pedal to throttle allows an input to be provided to the system to initiate movement of the vehicle from a stationary position.
Comparator 402 also closes switch 410 to transmit pedal position directly to the clutch pressure servo controlled 450. Hence, the degree of engagement of clutch 40 is proportional to pedal position up to the point where ratio R equals 4.7. During this period the degree of direct control of the accelerator pedal over throttle 12 diminishes as engine speed increases in accordance with the above-described relationship.
When ratio R drops below 4.7, switches 404 and 410 open, and comparator 411 closes switch 412 to deliver maximum pressure to the clutch servo controller 450. Maximum pressure causes full engagement of the clutch. As the vehicle accelerates beyond this point, it is under total automatic control.
It can be seen that if no start/neutral S/N switch were provided any depression of accelerator pedal 18 upon startup would cause engagement of clutch 40 and a forward lurch of the vehicle. The S/N switch therefore effectively disables the effect of .alpha. on clutch 40 to permit safe startup.
FIG. 9 relates primarily to the sheave pressure generator for the driven sheave 200 and the sheave pressure generator for the driver sheave 300. Pressure generator 200 includes circuitry which engages the transmission ratio to increase the load on the engine if the engine tends to exceed the maximum operating speed of 5500 rpm (N.sub.MAX). Also provided is circuitry for changing the transmission ratio to decrease the load on the engine should the engine speed tend to decrease below the idle speed of 1000 rpm (N.sub.MIN). This is accomplished by means of summing amplifiers 230, 232 and clipping circuits 234, 236. Summing amplifier 232 and clipping circuit 236 act to reduce pressure on the driven sheave 200 to increase the load on the engine. Amplifier 232 receives N.sub.E, applied to its negative input terminal, and N.sub.MAX, applied to its positive input terminal, and produces a summed output signal N.sub.MAX -N.sub.E. This summed output is applied to clipping circuit 236 which is a non-linear device having the characteristic shown in FIG. 9. This device can be, for example, a reverse biased diode which produces a negative substantially linear output for negative excursions of its input signal and a zero output for positive excursions.
Consequently, if N.sub.E exceeds N.sub.MAX, the input signal applied to circuit 236 will be negative, thereby resulting in a negative output signal. This negative output signal is then applied to summing amplifier 210 to reduce the value of its summed output signal in proportion to the amount N.sub.E exceeds N.sub.MAX. As a result, the pressure on driven sheave 200 will be proportionally decreased. On the other hand, if N.sub.E is less than N.sub.MAX, the input signal applied to clipping circuit 236 will be positive resulting in a zero output signal applied to amplifier 210. Such an output signal has no affect on the summed output signal of amplifier 210; thus, no change in the signal supplied to the driven servo-controlled 250 is produced.
Summing amplifier 230 and clipping circuit 234 act to increase pressure on the driven sheave 200 to decrease the load on the engine. Amplifier 230 receives N.sub.E, applied to its negative input terminal, and N.sub.MIN, applied to its positive input terminal, and produces a summed output signal N.sub.MIN -N.sub.E. This summed output is applied to a clipping circuit 234 similar to circuit 236. Circuit 234, however, has a non-linear transfer characteristics which produces a positive substantially linear output for positive excursions of its input signal and a zero output for negative excursions. Circuit 234 can be, for example, a forward baised diode. If N.sub.E falls below N.sub.MIN, the input signal applied to clipping circuit 234 will be positive, thereby resulting in a positive output signal. This positive output signal is then applied to summing amplifier 210 to increase the value of its summed output signal in proportion to the amount N.sub.E is less than N.sub.MIN. As a result, the pressure on driven sheave 200 will be proportionally increased. On the other hand, if N.sub.E is greater than N.sub.MIN, then a zero output signal will be produced by circuit 234 which has no affect on the summed signal applied to servo-controller 250.
Pressure generator 200 also includes circuitry for adjusting the sensitivity of accelerator pedal 18, depending on vehicle speed, to more closely simulate the "feel" of a conventional vehicle. This is required because of the inherent operating characteristics of the engine and CVT. That is, at higher vehicle speeds, the torque produced by the engine remains fairly high and constant (see FIG. 1). In the conventional vehicle the remaining small percentage of torque which can be extracted from the engine is delivered to the rear wheels through a transmission in high gear with a fixed, very low reduction ratio. Vehicle acceleration is therefore fairly insensitive to accelerator pedal movement at high speeds. In a CVT equipped vehicle, however, depression of the accelerator pedal even at high vehicle speeds results in an increased reduction ratio and an associated multiplication of torque in excess of that provided in the conventional vehicle. Thus, if only direct accelerator pedal position .alpha. were used to control CVT ratio at higher speeds, vehicle response would be extremely sensitive to accelerator pedal movement. The sensitivity of the accelerator pedal 18 must therefore be dulled at higher vehicle speeds.
Pedal sensitivity is controlled by two comparators 212, 214. As long as vehicle speed is below a threshold equivalent of drive shaft speed N.sub.DS equal to or less than 1173 rpm, switch 216 remains closed to deliver the .alpha. signal directly to amplifier 210. This effectively is torque control. When drive shaft speed N.sub.DS exceeds 1173 rpm, switch 216 opens and switch 218 is closed so that a pedal position signal equivalent to .alpha. divided by N.sub.DS (provided by divider 220) is delivered to amplifier 210. This effectively is power control. In this way, the effect of any movement of accelerator pedal 18 in the higher speed ranges is diminished so as to more closely simulate the pedal response of a conventional automobile.
FIG. 10 shows a modification of the sheave pressure generator 200, wherein accelerator sensitivity is controlled as a function of ratio R. Comparator 212' close switch 216' to connect the accelerator pedal position signal .alpha. directly to amplifier 210 when ratio R equals or exceeds 3. The comparator 214' closes switch 218' to feed a dulled signal to amplifier 210 from divider 220' when ratio R is below 3.
The control of transmission ratio described above actually is a ratio rate control, R. That is, the greater the magnitude of the increase (or decrease) in fluid pressure on driven sheave 20 commanded by accelerator pedal 18, the more rapid the change of sheave diameters will be. Thus, for example, a rapid depression of accelerator pedal 18 will result in a rapid change of CVT ratio and quick acceleration. This, of course, closely simulates the characteristics of a conventional vehicle.
The control system disclosed in FIGS. 4-11 involves, in part, the recognition that control of the ratio rate R of the CVT, rather than merely the CVT radio, yields improved CVT control. This improved control is explained by reference to the following derived vehicle performance equation: ##EQU1## where I.sub.EQ =I.sub.CDS +R.sup.2 I.sub.E,
R is the ratio rate of the transmission, PA1 R is the ratio of the transmission, PA1 I.sub.E is engine inertia, PA1 N.sub.E is engine speed, PA1 T.sub.E is engine torque, PA1 T.sub.RL is road load torque reflected to the drive shaft and includes tires, final drive and axle losses, PA1 T.sub.loss is transmission loss, PA1 I.sub.CDS is car inertia reflected to the drive shaft, and PA1 N.sub.DS is vehicle acceleration measured at the drive shaft.
It is clear that the acceleration of the vehicle N.sub.DS is dependent primarily upon control of any one or more of these variable such as, for example, T.sub.E, R or R. Generally, conventional vehicle systems vary the transmission ratio R and engine output torque T.sub.E to provide the required transmission and vehicle control. By controlling R, however, it is difficult to constantly maintain engine torque and speed along the ideal operating line. This is due to the fact that each time R is varied, the load on the engine is changed which, in turn, affects the engine's output torque and vehicle acceleration.
Attempts to simultaneously change the engine torque and speed to force engine operation back on the ideal line have necessitated very complex control systems, since control is dependent on several variables of the performance system. For example, these systems must necessarily perform the complicated task of calculating the necessary target throttle position and CVT ratio R to force engine operation back on the ideal line. These systems also require the calculation of ratio rate R so that the rate is changing the ratio to the target value does not result in undesirable vehicle dynamics. For example, if R is selected to be excessive then an undesirable deceleration of the vehicle will occur before the vehicle can accelerate. This phenomenon results from the negative sign of the R term in the above performance equation.
This control system, however, recognizes that R can easily be sensed and controlled without causing the other variables to adversely affect engine performance. This is accomplished by separating the engine control from the transmission control so that engine torque and speed are fixed along the ideal engine operating line. As a result of controlling R no adverse affect on the other dependent variables occur. In particular, changing R along, with its concomitant change on R, will not force engine operation off the ideal operating line since engine speed and torque are determined soley by the fuel function f(N.sub.E). As a result, vehicle acceleration N.sub.DS and output torque T.sub.O are controlled solely by ratio rate R, rather than by the other variables of the performance system.
It has been discovered that rate of change of ratio (R) is closely approximated by the following relationships: ##EQU2## In the V-belt traction drive CVT discussed above, the comparison of accelerator pedal position .alpha. and output torque T.sub.O occurs inherently in the belt and pulley components to effect a ratio change at a rate R. Other types of CVTs may require different control elements to effect this relationship. As stated earlier, however, other parameters indicative of system performance may be used to effect a ratio change at a rate R, where R is proportional to the difference between the desired performance parameter and the actual measured performance parameter.
The above described control scheme is graphically illustrated in FIG. 11. FIG. 11 is a plot of engine speed N.sub.E as a function of vehicle speed or drive shaft speed N.sub.DS. The minimum and maximum CVT ratios are illustrated by the straight lines emanating from the origin of the graph. The idle speed (N.sub.MIN =1000 rpm) is indicated by a lower horizontal line, while the maximum permissible engine speed (N.sub.MAX =5500 rpm) is indicated by an upper horizontal line. The maximum vehicle speed is defined by a vertical line at the right hand edge of the graph.
The graph of FIG. 11 is divided into a number of discrete operating regions. "A" designates the normal region of operation of the engine-CVT system. Region "A" is bounded by the line maximum CVT ratio, the line of maximum engine speed, the line of maximum vehicle speed, the line of minimum CVT ratio and the idle speed line. During operation of the system in region "A", clutch 40 is fully engaged and throttle position is wholly a function of engine speed in accordance with the fuel function f(N.sub.E). Operation to the left of the dashed verticle line indicating a drive shaft speed of 1173 rpm is under torque control, while operation to the right of this line is under power control (see the above two equations, and the accelerator pedal sensitivity circuitry illustrated in FIGS. 9 and 10). Region "B" is the region of start-up control, that is, the operation of the engine-CVT system during slow vehicle operation when clutch 40 is only partially engaged. The control for this operation (400) is illustrated in FIG. 8.
Operation of the engine-CVT system is in the remaining three regions "C", "D" and "E" is effectively prevented by the above described control system. That is, operation in region "C" is prevented by the physical limitation of minimum CVT ratio, and by the fuel diminishing circuits comprising fuel diminishing valve 11, pulse width modulator 110, differentiator 112 and fuel diminishing comparator 114 of engine control circuit 100 (FIG. 7). Region "D" is the region of overspeed control, governed by the fuel suspension mechanism 9 and fuel suspension comparator 108 of engine control circuit 100 (FIG. 7), and by amplifier 232 and clipping circuit 236 of sheave pressure generator 200 (FIG. 9). Region E is the region of engine idle control which is governed by amplifier 230 and clipping circuit 234 of sheave pressure generator 200 (FIG. 9).
Also shown on the graph of FIG. 11 is a load line which indicates the engine speed required to maintain any constant vehicle speed along a level road. The term "load" includes road load, final drive losses and the like, and represents the actual load on the engine-CVT system. In order for the control scheme of the invention to function only in accordance with the fuel function so as to maintain engine operation along the ideal operating line, it is desirable that the CVT ratio range include substantially all ratios required to maintain constant vehicle speed for any normally encountered load. That is, the minimum CVT ratio preferably is smaller than that required to maintain constant vehicle speed along a level road, and the maximum CVT ratio preferably is greater than that required to maintain constant vehicle speed up the steepest grade which one might expect to encounter. This relationship is graphically illustrated by the physical location of the load line in the graph of FIG. 11 above the line of minimum CVT ratio in region "A". All other load lines should lie below the line of maximum CVT ratio. A desirable CVT ratio range for accomplishing this is approximately 11:1 with, for example, a maximum CVT ratio of 22:1 (total vehicle ratio, including final drive ratio), and a minimum CVT ratio of 2:1. A transmission having such a wide ratio range is disclosed in commonly assigned application Ser. No. 290,293, filed Aug. 5, 1981. Of course, a CVT having a smaller ratio range would be operable, but would not have as much flexibility as one with a wider range.
Referring to FIG. 6, the mechanics of a change in CVT ratio now will be described with reference to the axial forces produced by the pressurized fluid in chambers 26 and 36. The lower curve in FIG. 6 is a plot of steady state axial force on movable portion 24 of driven sheave 20 as a function of CVT ratio. Similarly, the upper curve is a plot of steady state axial force tending to resist inward movement of movable portion 34 as a function of CVT ratio. As described below, when for example a signal is generated to increase the ratio of the CVT from 1.0 to approximately 1.7, the fluid pressure in chamber 26 is increased to raise the axial force from approximately 175 kg. to, ultimately, approximately 270 kg. Movable portion 24 does not move instantaneously, however, due to the inertia of the system. Accordingly, the curve which represents the transient change taking place in sheave 20 is defined by movement from point A to point B at a constant ratio of 1.0, and then to point C where equilibrium is reached. Correspondingly, an increase in pressure in chamber 36 of driver sheave 30 results in an increase in axial force of movable portion 34 of sheave 30 from approximately 315 kg. (point D) to approximately 380 kg. (equilibrium point E). Despite this increase in axial force, the increase tension on belt 15 occasioned by expansion of the diameter of sheave 20 forces the two portions 32, 34 of sheave 30 apart so that sheave 30 has a smaller driving diameter. Driver sheave 30, therefore, follows in a controlled manner any changes occurring to driven sheave 20.
Sheave pressure generator 300 geneates a pressure appropriate for driver sheave 30 as a function of ratio R and measured output torque T.sub.O. This function has been found to satisfactorily tension belt 15, without undue stress, and effect a smooth change of ratio. And example of a function suitable for this purpose is as follows: ##EQU3## there P.sub.DR is the fluid pressure in chamber 36 of driver sheave 30, and K.sub.1, K.sub.2 and K.sub.3 are appropriately selected constants.